Question

My student was messing around with the bunsen burner when he noticed the following. He had put a wire gauze on top of the burner before turning the gas on. When he used the lighter on the bottom portion i.e. below the wire gauze, a flame was only visible at that portion but not above the wire gauze. When he used the lighter on the portion above the wire gauze, the flame only appeared on the top portion but not the bottom portion! Why?

Asked by: Pam

 Answer

What a great observation! I shall have to show this to my students and see what they think about it! In all my years of teaching I have never even thought of this. Even more surprising, neither have any of my thousands of students!

Well, what could it be about the wire gauze that might explain this wonderful event? Wire is a great heat sink! So, think about why a gas burns. It has to have enough thermal energy to begin the chemical reaction. If you light the gas below the gauze the lighter provides the energy to begin the burning. But the gauze takes away the thermal energy that would otherwise be there to ignite the gas above the gauze. If you light the gas above the gauze the same thing happens. Either way, the gauze, acting as a heat sink, removes the thermal energy from the system so that the temperature above or below it is not sufficient to ignite the gas.

I hope that if you see this or a better answer to this question, that you will challenge your students to think about the properties of the wire gauze and the necessary components that must be present for a gas to burn. See if any of them can come up their own answer. Their Joy of Discovery is priceless! It is also, as you well know, far more memorable than simply reading the answer provided by someone else!

Answered by: Tom Young, M.S., Science Teacher, Whitehouse High School, Texas

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